Mobile electronic devices that are portable outdoors, such as mobile phones, notebook computers, and tablet computers, may include acoustic components, such as a microphone and a speaker. In assembly of such an acoustic component to a housing, a compressible member made of a compressible material, such as rubber or polyurethane foam, may be placed between the housing and the acoustic component or a substrate for mounting the acoustic component, so that the acoustic component is assembled while the compressible member is being compressed. By assembling the acoustic component in this manner, it is possible to cut off an acoustic component chamber containing the acoustic component from the other space in the housing, and improve airtightness of the acoustic component chamber. This makes it possible to reduce leakage of sound from the acoustic component chamber, reduce entry of noise from the other space in the housing into the acoustic component chamber, and improve sensitivity of the acoustic component.
In a mobile electronic device including an acoustic component as described above, a device case (housing) is provided with a sound hole which allows input of sound from the outside into the acoustic component and output of sound to the outside. The sound hole is covered with a protective sheet having a waterproof and dustproof function.
A related technique has been proposed in which a sound hole in a housing is covered with a porous protective sheet, so as to ensure breathability (gas permeability) and waterproof and dustproof performance. However, for example, if the electronic device has been dropped in water many times, clogging or crystallization may occur in micropores of the porous protective sheet. For example, if the porous protective sheet is clogged with a surface-active agent or the surface-active agent is crystallized, the surface-active agent may be dissolved when the electronic device is dropped in water later on. As a result, the waterproof function of the porous protective sheet may be lost. Crystallization of seawater may also cause loss of the waterproof function of the porous protective sheet.
Related technique is disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2007-37189, 2010-41671, and 2008-278064.
To ensure the waterproof function even if the electronic device has been dropped in water many times, the sound hole in the device case may be covered with a non-breathable protective sheet. However, in this instance, there is nowhere for air in the acoustic component chamber to escape.
Therefore, the resulting increase in internal pressure of the acoustic component chamber during assembly of the acoustic component to the housing may damage, for example a diaphragm (vibrating plate) of the acoustic component. The increased pressure in the acoustic component chamber may cause displacement of the diaphragm of the acoustic component, degrade sensitivity of the acoustic component, and lead to production variations.